TTYs (also known as TDDs) are text terminals that people with hearing impairments use in order to communicate over telephone lines. There is no single TTY communication protocol. Three of the more commonly used protocols are the United States standard Baudot 45, the United Kingdom standard Baudot 50, and Turbo Code protocol. Unlike computer modems and fax machines, none of the three TTY protocols listed above have a “handshake” tone or a carrier tone. In other words, TTYs are silent when not transmitting. This assures that the transmission of a TTY is not interrupted for lack of a handshake tone or carrier tone. However, this means that there is no mechanism by which a telephony system can automatically identify a TTY use, much less the protocol being used, until after a user has begun typing.
A further complication is that nearly half of all TTY use is actually mixed-mode voice and TTY dialog. A common type of mixed-mode use is by individuals with moderate hearing impairments, who never the less are able to speak clearly. These people often prefer to receive information with their TTYs, and then, speak in response. Another common pattern is for individuals to alternate between TTY and voice on the same call, relying on voice for informal conversation and TTY for critical information such as credit card numbers and bank balances. (The need to intermix voice and TTY on the same call is a reason why a handshake-free, carrier-free modem protocol is used for TTY communication.)
In the United States, Section 508 of the Workforce Investment Act of 1998, as well as section 251(a)(2) and 255 of the Telecommunication Act of 1996, requires telephony systems to be TTY compatible. The Federal Communication Commission (FCC) requirement for system performance is that TTY character error rate (i.e., the percentage of transmitted TTY characters that are displayed improperly by the receiving device) must be 1% or less. When tones emitted by TTYs are transmitted via voice channels on VoIP systems, this FCC-mandated level of performance is virtually impossible to achieve. One problem is that some of the low-data-rate audio encoders commonly used in VoIP systems cannot encode TTY tones accurately. A problem that may be more difficult to solve is the one caused by packet loss. Illustratively, when using United States standard TTY protocol on a call in a VoIP system in which a TTY-compatible audio encoder is used (such as G.711) and with a 20 millisecond packet length, the 1% error rate required by the FCC is exceeded when the packet loss exceeds only 0.12%—a packet loss rate far lower than is normal in typical VoIP systems. A similar problem exists in cellular telephone transmissions. The problem is further complicated by the mixed-mode voice and TTY dialog.
In the prior art, it has been recognized that during packet transmission of TTY signals that the signals can be converted to a digital form and transmitted as digital information in a more reliable digital channel. U.S. Pat. No. 6,351,495 discloses a cellular transmission system where TTY audio signals are recognized and converted to digital information for transmission via the signal transmission portion of the cellular system. However, the system disclosed in U.S. Pat. No. 6,351,495 does not prevent the loss of either audio or TTY information during the transition from one transmission mode to the other transmission mode. Since nearly half of all TTY use is the mixed-mode voice and TTY dialog, this represents a serious problem in the prior art methods of transmitting TTY information over transmission systems that are based on packets. Also, in the prior art for VoIP systems, it is known to encode precision tones such as multi-frequency dial tones as digital information and transport this digital information to a destination point via a signaling channel that is distinct from the bearer channel used for encoded voice. One such VoIP system that transports multi-frequency dial tones in this manner is disclosed in U.S. patent application Ser. No. 09/18,909, entitled “Integration of Remote Access and Service”, filed Nov. 22, 2000, and assigned to the same assignee as the present patent application. U.S. patent application Ser. No. 09/18,909 is hereby incorporated by reference.